Published online by Cambridge University Press: 01 January 1999
The projective moduli variety [Sscr ][Uscr ]C(2) of semistable rank 2 vector bundles with trivial determinant on a smooth projective curve C comes with a natural morphism ϕ to the linear series [mid ]2×Θ[mid ] where Θ is the theta divisor on the Jacobian of C. Well- known results of Narasimhan and Ramanan say that ϕ is an isomorphism to P3 if C has genus 2 [16], and when C is nonhyperelliptic of genus 3 it is an isomorphism to a special Heisenberg-invariant quartic QC⊂P7 [18]. The present paper is an attempt to extend these results to higher genus.